feat: add strip_option(fallback = field_opt)

[ifiokjr]

Description

Fixes #117 using the API described in #117 (comment).

#[derive(TypedBuilder)]
struct SomeStruct {
     #[builder(default, setter(strip_option)]
     inner: Option<u32>,
     #[builder(setter(strip_option(fallback="outer_opt"))]
     outer: Option<u32>,

}

/// `strip_option` retains its current behaviour avoiding a breaking change.
/// `strip_option(fallback ="outer_opt")` strips the `Option`, and creates a fallback that still takes the option. 
/// This makes it fully opt-in.
fn new_api(value: Option<u32>) -> SomeStruct {
    match value {
        None => SomeStruct::builder().outer_opt(None).build(),
        Some(v) => SomeStruct::builder().inner(v).outer(v).build(),
    }
}

/// `strip_option(fallback ="outer_opt") makes the original `outer` builder method take the value, and creates an `outer_opt` which still takes an `Option`.
fn new_way(value: Option<u32>) -> SomeStruct {
    SomeStruct::builder().outer_opt(value).build()
}

[ifiokjr]

@idanarye maybe I can extend the functionality here to also cover strip_bool.

[idanarye]

I have some remarks on this. Do you want to fix them? If not, I don't mind doing them myself after merging.

[ifiokjr]

@idanarye should I create a separate PR for strip_bool?

[idanarye]

Yes, but let's merge this one first. Much of it would probably be the same so lets iron it out once instead of working on the same issues twice.

[ifiokjr]

Yes, but let's merge this one first. Much of it would probably be the same so lets iron it out once instead of working on the same issues twice.

I'll create a follow up PR once this is merged.